William L. Graf

Dam nation: A geographic census of American dams and their large-scale hydrologic impacts

Newly available data indicate that dams fragment the fluvial system of the continental United States and that their impact on river discharge is several times greater than impacts deemed likely as a result of global climate change. The 75,000 dams in the continental United States are capable of storing a volume of water almost equaling one years mean runoff, but there is considerable geographic variation in potential surface water impacts. In some western mountain and plains regions, dams can store more than 3 years runoff, while in the Northeast and Northwest, storage is as little as 25% of the annual runoff. Dams partition watersheds; the drainage area per dam varies from 44 km 2 (17 miles 2 ) per dam in New England to 811 km 2 (313 miles 2 ) per dam in the Lower Colorado basin. Storage volumes, indicators of general hydrologic effects of dams, range from 26,200 m 3 km 22 (55 acre-feet mile 22 ) in the Great Basin to 345,000 m 3 km 22 (725 acre-feet mile 22 ) in the South Atlantic region. The greatest river flow impacts occur in the Great Plains, Rocky Mountains, and the arid Southwest, where storage is up to 3.8 times the mean annual runoff. The nations dams store 5000 m 3 (4 acre-feet) of water per person. Water resource regions have experienced individualized histories of cumulative increases in reservoir storage (and thus of downstream hydrologic and ecologic impacts), but the most rapid increases in storage occurred between the late 1950s and the late 1970s. Since 1980, increases in storage have been relatively minor.

Fluvial adjustments to the spread of tamarisk in the Colorado Plateau region

Tamarisk, a shrub or low tree that was artificially introduced into the American Southwest in the late 1800s, has spread throughout the Colorado Plateau region by occupying islands, sand bars, and beaches along streams. Historical photographs show that tamarisk spread from northern Arizona to the upper reaches of the Colorado and Green Rivers at a rate of about 20 km/yr. Detailed studies on the Green River in Canyonlands National Park, Utah, show that the plant has trapped and stabilized sediment, causing an average reduction in channel width of 27%. Photogrammetric analysis of historical ground photography, including photos from John Wesley Powell9s 1871 expedition, and recent aerial photographs supplemented by field surveys provided quantitative data. Expanded islands and channel-side bars exhibit allometric relationships as they change, apparently maintaining a balance between turbulence and friction. Overbank flooding is common on the tamarisk-stabilized features.

Damage Control: Restoring the Physical Integrity of America’s Rivers

Technological development of America’s rivers, including the installation of more than 80,000 dams, has segmented the streams and fragmented their watersheds. A vision for the nation’s rivers requires science and public policy that emphasize restoration and maintenance of the rivers’ physical integrity to create a great river legacy for future generations. The Clean Water Act mandates the biological, chemical, and physical integrity of the nation’s rivers, but researchers and decision makers have paid scant attention to physical integrity. Physical integrity for rivers refers to a set of active fluvial processes and landforms wherein the channel, near-channel landforms, sediments, and overall river configuration maintain a dynamic equilibrium, with adjustments not exceeding limits of change defined by societal values. Rivers with physical integrity have functional surfaces and materials that are susceptible to monitoring and measurement with a set of geographic indicator parameters. Science and policy for the nation’s rivers must blend watershed principles with ecosystem concepts, focus on change rather than equilibrium as a defining characteristic of streams, adopt probabilistic rather than exclusively deterministic approaches, and pursue geographic representativeness through hydrodiversity, geodiversity, and biodiversity. The dams that fragment the system also offer opportunities for restoration of some natural characteristics through adjusted operating rules, redesign, and physical renovation, along with the removal of some dysfunctional structures. In the near future, when social values for rivers are likely to revolve around protection for endangered species, economics of flood protection, and dam removal issues, we can enhance restoration efforts by including physical integrity in research agendas, policy decisions, operational rulemaking, and public debate. Our multicentury legacy for future generations can and should be to establish physical integrity for rivers that are as natural as possible, thus insuring that as a system they are parts of the infrastructure for a vibrant national economy, continuing threads of our cultural heritage, and quality natural environments.

Reclaiming freshwater sustainability in the Cadillac Desert

Increasing human appropriation of freshwater resources presents a tangible limit to the sustainability of cities, agriculture, and ecosystems in the western United States. Marc Reisner tackles this theme in his 1986 classic Cadillac Desert: The American West and Its Disappearing Water. Reisners analysis paints a portrait of region-wide hydrologic dysfunction in the western United States, suggesting that the storage capacity of reservoirs will be impaired by sediment infilling, croplands will be rendered infertile by salt, and water scarcity will pit growing desert cities against agribusiness in the face of dwindling water resources. Here we evaluate these claims using the best available data and scientific tools. Our analysis provides strong scientific support for many of Reisners claims, except the notion that reservoir storage is imminently threatened by sediment. More broadly, we estimate that the equivalent of nearly 76% of streamflow in the Cadillac Desert region is currently appropriated by humans, and this figure could rise to nearly 86% under a doubling of the regions population. Thus, Reisners incisive journalism led him to the same conclusions as those rendered by copious data, modern scientific tools, and the application of a more genuine scientific method. We close with a prospectus for reclaiming freshwater sustainability in the Cadillac Desert, including a suite of recommendations for reducing region-wide human appropriation of streamflow to a target level of 60%.

The Big Questions in Geography

In noting his fondness for geography, John Noble Wilford, science correspondent for The New York Times, nevertheless challenged the discipline to articulate those big questions in our field, ones that would generate public interest, media attention, and the respect of policymakers. This article presents our collective judgments on those significant issues that warrant disciplinary research. We phrase these as a series of ten questions in the hopes of stimulating a dialogue and collective research agenda for the future and the next generation of geographic professionals.

Rapids in Canyon Rivers

Evidence from river surveys, historical photographs, and field investigations show that 410 rapids in 12 canyon rivers of the Colorado Plateau region are distributed randomly or only slightly more regularly than random. Rapids, in their distribution, show little tendency toward equal spacing, are not related to discharge, and are not always colocated with debris sources. Analysis of force of flowing water and resistance of the boulders in rapids of the canyons of the Green River in Dinosaur National Monument, Utah-Colorado, shows that although some particles may have been moved by flood flows within the past 75-100 years, the largest particles were not moved. Under the present climatic/hydrologic regime, these rapids are relict geo-morphic features that are either unchanging or are accumulating debris from tributary sources. Consequently, any theory attempting to explain the distribution and dynamics of rapids in canyon rivers must (1) appeal to the relationship between force and resistance as a measure of stability at particular sites, without reference to operation of the river system as a whole; and (2) account for the effects of climatic/hydrollogic conditions that have recurrence intervals greater than 100 years.

Aging Infrastructure and Ecosystem Restoration

Targeted decommissioning of deteriorated and obsolete infrastructure can provide opportunities for restoring degraded ecosystems.

The Geomorphology of the Glacial Valley Cross Section

Several alpine valley systems in the southeastern Beartooth Mountains, Montana and Wyoming, have been examined using techniques similar to methods of stream system analysis. The general equation y ...

Late Holocene sediment storage in canyons of the Colorado Plateau

Field surveys of ten drainage basins 10 to 1,790 km2 in area in the Colorado Plateau (Utah and Arizona) and repeat photography of 156 fluvial environments providing comparative evidence spanning up to 114 yr reveal that two stratigraphic units characterize the alluvial materials on valley floors. An older unit deposited generally between 1250 A.D. (±200 yr) and 1880 A.D. (±20 yr) was dissected by arroyo development by the early twentieth century. A younger unit was deposited generally during the period after approximately 1943 (±5 yr). Rates of sedimentation in the 1943–1980 period were almost twice the rates in the 1250–1880 period. Channel-storage rates during the two periods of sedimentation are related to drainage-basin area by a power function similar to functions relating slope-sediment production and sediment yield to drainage basin area. Storage rates in the 1943–1980 period were about 50% of the rates of slope-sediment production. The amount of sediment stored in the two units exhibits a definable spatial pattern. Source streams (with basin areas upstream 10,000 km2) alternatively stored or evacuated moderate amounts of sediment in response to upstream evacuations of sediment from the Colorado Plateau, riparian vegetation change, and hydro-climatic changes in mountain watersheds outside the plateau region.

Tamarisk and river-channel management

Tamarisk (Tamarix chinensis, Lour.) an artificially introduced tree, has become a most common species in many riparian vegetation communities along the rivers of the western United States. On the Salt and Gila rivers of central Arizona, the plant first appeared in the early 1890s, and by 1940 it grew in dense thickets that posed serious flood-control problems by substantially reducing the capacities of major channels. Since 1940 its distribution and density in central Arizona have fluctuated in response to combined natural processes and human management. Groundwater levels, channel waters, floods, irrigation return waters, sewage effluent, and sedimentation behind retention and diversion works are major control mechanisms on the growth of tamarisk; on a regional scale of analysis, groundwater levels are the most significant under present conditions.